JUN 03, 2021 7:19 AM PDT

Why did all the sharks die?

New research published in the journal Science reports on the discovery of a massive shark die-off - nearly 19 million years ago. Back then, explains lead author of the study Elizabeth Sibert, there was over tenfold the number of sharks in the oceans that we have today. Sibert, a Hutchinson postdoctoral associate in Yale's Department of Earth and Planetary Sciences and the Yale Institute for Biospheric Studies, says she and her team discovered the extinction “almost by accident.”

"I study microfossil fish teeth and shark scales in deep-sea sediments, and we decided to generate an 85-million-year-long record of fish and shark abundance, just to get a sense of what the normal variability of that population looked like in the long term," Sibert said. "What we found, though, was this sudden drop-off in shark abundance around 19 million years ago, and we knew we had to investigate further."

That there were no known climate or ecosystem shifts or disasters at the time of the die-off makes the discovery even more of an enigma."This interval isn't known for any major changes in Earth's history," notes Sibert, "yet it completely transformed the nature of what it means to be a predator living in the open ocean,” adding that over 70% of the world's sharks died off at this time. 

The researchers report that this die-off represents a level of extinction twice that seen during the Cretaceous-Paleogene mass extinction event 66 million years ago that eliminated three-quarters of the plant and animal species on Earth. In other words - it was a huge event, and yet we had no idea about it and now have no idea why it occurred.

The implications of the study don’t just reach into our planet’s past, but extend toward the future of our current ecosystems. "The current state of declining shark populations is certainly cause for concern and this paper helps put these declines in the context of shark populations through the last 40 million years," said co-author Leah Rubin. "This context is a vital first step in understanding what repercussions may follow dramatic declines in these top marine predators in modern times," noting that it is still unclear why shark populations did not rebound to their original abundance. 

"This work could tip-off a race to understand this time period and its implications for not only the rise of modern ecosystems, but the causes of major collapses in shark diversity," comments Pincelli Hull, an assistant professor of Earth and planetary science at Yale. "It represents a major change in ocean ecosystems at a time that was previously thought to be unremarkable."

Sources: Science, Science Daily

About the Author
BA Environmental Studies
Kathryn is a curious world-traveller interested in the intersection between nature, culture, history, and people. She has worked for environmental education non-profits and is a Spanish/English interpreter.
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